Aluminum and its alloys have found a variety of industrial applications. However, because of the reactivity of aluminum and its alloys, and their tendency toward corrosion and environmental degradation, it is necessary to provide the exposed surfaces of these metals with an adequate corrosion-resistant and protective coating. Further, such coatings should resist abrasion so that the coatings remain intact during use, where the metal article may be subjected to repeated contact with other surfaces, particulate matter and the like. Where the appearance of articles fabricated is considered important, the protective coating applied thereto should additionally be uniform and decorative.
In order to provide an effective and permanent protective coating on aluminum and its alloys, such metals have been anodized in a variety of electrolyte solutions, such as sulfuric acid, oxalic acid and chromic acid, which produce an alumina coating on the substrate. While anodization of aluminum and its alloys is capable of forming a more effective coating than painting or enameling, the resulting coated metals have still not been entirely satisfactory for their intended uses. The coatings frequently lack one or more of the desired degree of flexibility, hardness, smoothness, durability, adherence, heat resistance, resistance to acid and alkali attack, corrosion resistance, and/or imperviousness required to meet the most demanding needs of industry.
Heat resistance is a very desirable feature of a protective coating for aluminum and its alloys. In the cookware industry, for instance, aluminum is a material of choice due to its light weight and rapid heat conduction properties. However, bare aluminum is subject to corrosion and discoloration, particularly when exposed to ordinary food acids such as lemon juice and vinegar, as well as alkali, such as dishwasher soap. PTFE or silicone containing paints are a common heat resistant coating for aluminum, which provide resistance to corrosion, discoloration and give a “non-stick” cooking surface. However, PTFE containing paints have the drawback of insufficient adherence to the substrate to resist peeling when subjected to abrasion. To improve adherence of PTFE coatings, manufacturers generally must provide three coats of paint on the aluminum substrate: a primer, a midlayer and finally a topcoat containing PTFE. This three-step process is costly and does not solve the problem of insufficient abrasion resistance and the problem of subsequent corrosion of the underlying aluminum when the protective paint, in particular the PTFE coating wears off. Likewise, the non-stick silicone coatings eventually wear away and the underlying aluminum is exposed to acid, alkali and corrosive attack.
To improve toughness and abrasion resistance, it is known in the cookware industry to anodize aluminum to deposit a coating of aluminum oxide, using a strongly acidic bath (pH<1), and to thereafter apply a non-stick seal coating containing PTFE. A drawback of this method is the nature of the anodized coating produced. The aluminum oxide coating is not as impervious to acid and alkali as oxides of titanium and/or zirconium. Articles coated using this known process lose their PTFE coatings with repeated exposure to typical dishwasher cycles of hot water and alkaline cleaning agents.
So called, hard anodizing aluminum results in a harder coating of aluminum oxide, deposited by anodic coating at pH<1 and temperatures of less than 3° C., which generates an alpha phase alumina crystalline structure that still lacks sufficient resistance to corrosion and alkali attack.
In another known attempt to provide a corrosion-, heat- and abrasion-resistant coating to support adherence of PTFE to aluminum, an aluminum alloy was thermally sprayed with titanium dioxide to generate a film that is physically adhered to the underlying aluminum. This film had some adherence to the aluminum article, but showed voids in the coating that are undesirable.
Thus, there is still considerable need to develop alternative anodization processes for aluminum and its alloys which do not have any of the aforementioned shortcomings and yet still furnish adherent, corrosion-, heat- and abrasion-resistant protective coatings of high quality and pleasing appearance.